Biomolecular arrays (such as DNA, RNA, peptide, or protein microarrays) are known and are used, for example, as diagnostic or screening tools. Such microarrays include regions of usually different sequence biomolecules (such as polynucleotides or polypeptides) arranged in a predetermined configuration on a substrate. These regions (sometimes referenced as “array features”) are positioned at respective locations (“addresses”) on the substrate. Biomolecular arrays typically are fabricated on planar substrates either by depositing previously obtained biomolecules onto the substrate in a site specific fashion or by site specific in situ synthesis of the biomolecules upon the substrate. The microarrays, when exposed to a sample, will undergo a binding reaction with the sample and exhibit an observed binding pattern. This binding pattern can be detected upon interrogating the microarray. For example all biomolecule targets (for example, DNA) in the sample can be labeled with a suitable label (such as a fluorescent compound), and the label then can be accurately observed (such as by observing the fluorescence pattern) on the microarray after exposure of the microarray to the sample. Assuming that the different biomolecule targets were correctly deposited in accordance with the predetermined configuration, then the observed binding pattern will be indicative of the presence and/or concentration of one or more components of the sample.
Alternative methods of sensing binding events are being developed. One such method involves electrical detection of binding to probe DNA bound to a surface between two electrodes. See Park et al., Science (2002) 295:1503-1506. Park et al. have reported using a sandwich hybridization to close microelectrode gaps to detect binding of target DNA to probe DNA. In the reported method, probe DNA is immobilized between two microelectrodes spaced 20 microns apart, and then target DNA is hybridized to the probe DNA. Park et al. complete the sandwich hybridization using a third DNA molecule conjugated to gold nanoparticles. To complete the process, a silver stain is applied, resulting in bridging of the gap between the two microelectrodes with deposited silver. The probe/target binding event is then detected as a drop in resistance from the open circuit (>200 megohms) to a closed circuit (<˜500 ohms).
The sandwich hybridization method used by Park et al. requires that the target DNA have two recognition elements: one recognition element that is complementary to the probe DNA, and another recognition element that is complementary to the third DNA molecule (conjugated to the gold nanoparticle). This requirement for two recognition elements in the target DNA potentially limits the utility of the method. Additionally, the silver stain used to develop the deposited gold nanoparticles was observed to cause the particles to grow at different rates; this was interpreted as resulting from catalysis of silver deposition by the already deposited silver. Also, during the developing reaction, the silver solution had to be replaced every 2 to 3 minutes to avoid formation of silver particulates in solution. This may be inconvenient in terms of practical use of the method.
U.S. Pat. No. 6,403,317 to Anderson, granted Jun. 11, 2002 describes a method of detecting locations on a nucleic acid probe microarray at which hybridization occurs between targets in a fluid sample and nucleic acid probes disposed on a surface of the nucleic acid probe microarray. The referenced patent describes measuring the temperature at a plurality of discreet locations on the surface of the probe microarray while applying an oscillating level of energy to the probe microarray, thereby causing the temperature of the probe microarray to oscillate.
U.S. patent application Ser. No. 09/915,044, filed on Jul. 24, 2001, teaches the use of a redox active moiety that is associated with an electrode to detect, by means of the electrode, corresponding responses produced as a result of the activation of the redox active moiety. The magnitude of the corresponding response indicates the presence or absence of the target molecule in the sample.
What is needed is an improved method of detection of binding events that provides increased ease of use and utility.